1. Technical Field
The present invention relates to antigenic polypeptides and proteins, related vaccines and methods useful to induce an immune response which is protective to reduce the severity or prevent infection by ehrlichial parasites of the species Anaplasma marginale. 
2. Background
Anaplasmosis is a tick-borne disease of cattle caused by the obligate intraerythrocytic ehrlichial pathogen A. marginale. The acute phase of the disease is characterized by severe anemia, weight loss, fever, abortion, lower milk production and often death. The only known site of development of A. marginale in cattle is within bovine erythrocytes. The number of infected erythrocytes increases logarithmically and removal of these infected cells by phagocytosis results in development of anemia and icterus without hemoglobinemia and hemogloinuria. Biological transmission of A. marginale is effected by feeding ticks, while mechanical transmission occurs when infected blood is transferred to susceptible animals by biting flies or by blood-contaminated fomites. Cattle that recover from acute infection remain persistently infected and serve as reservoirs for mechanical transmission and infection of ticks. Approximately 20 species of ticks have been incriminated as vectors worldwide. The developmental cycle of A. marginale in ticks is complex and coordinated with the tick feeding cycle. After infection and development of A. marginale in tick gut cells, many other tick tissues become infected, including the salivary glands from where the ehrlichia is transmitted to vertebrates during feeding.
MSP1 is one of six major surface proteins (MSPs) that have been described on A. marginale from bovine erythrocytes and has been found to be conserved on tick salivary gland-derived A. marginale [1]. MSP1 is a heterodimer composed of two structurally unrelated polypeptides: MSP1a which is encoded by a single gene, msp1α, and MSP1b which is encoded by at least two genes, msp1β1 and msp1β2 [2, 3]. MSP1a is variable in molecular weight among geographic isolates because of a variable number of tandem 28 or 29 amino acid repeats in the amino terminal of the protein. Immunization of cattle with affinity-purified native MSP1 complex has previously been shown to induce protective immunity in cattle that received homologous or heterologous challenge with A. marginale geographic isolates [4, 5]. In addition, MSP1a and MSP1b expressed by recombinant Escherichia coli were shown to be putative adhesins to bovine erythrocytes [6, 7]. Although the MSP1 complex has been suggested to be involved in erythrocyte invasion, its role in infection and multiplication of the parasite in the tick vector has not been reported.
Recently, A. marginale has been grown in continuous culture in a cell line, IDE8, derived from embryos of the tick Ixodes scapularis [8]. See also U.S. Pat. No. 5,869,335, incorporated herein by reference. The Virginia isolate of A. marginale was initially propagated in the IDE8 tick cell line but subsequently an Oklahoma isolate was propagated in the tick cell line and characterized [9]. Colonies of A. marginale in cultured tick cells were morphologically similar to those observed in ticks [8-10], and A. marginale harvested from cell culture were infective for both cattle and ticks. All 6 MSPs of A. marginale were found to be conserved on the cell culture-derived organisms and the antigenic composition remained the same after successive passage in cell culture. The A. marginale isolate antigenic identity, as determined by the molecular weight of the MSP1a, was retained in culture [9, 11].
MSP1 complex isolated A. marginale also has been utilized for the vaccination of ruminants, as disclosed by McGuire et al. in U.S. Pat. No. 5,549,898 issued Aug. 27, 1996 (said patent being incorporated herein by reference).
Existing vaccines, however, including formulations using partially purified parasites from infected erythrocytes and cultured tick cells, are currently handicapped by mechanisms developed by the parasite to hide the most relevant epitopes from the host immune system. The inclusion of recombinant protein preparations in vaccine formulations would allow the development of host immune response against relevant epitopes not available for the host immune system in natural conformations present in whole-parasite or parasite-derived purified antigens.